1. Field of the Invention
The present invention relates generally to a security valve for providing a vent to the atmosphere upon power failure, ( a volume above a piston whose lower side is at vacuum, and more particularly to a vacuum security valve which includes a buffer volume in the piston.
2. Descriprion of the Prior Art
A vacuum security valve is a two way valve designed to address the following two problems associated with power failure: (a) when a mechanical vacuum pump stops, pump oil and vapor can back up into the chamber damaging components in the process; and (b) restarting a mechanical vacuum pump under vacuum imposes high starting loads which can blow motor circuit breakers or cause damage to the motor or the belts.
Existing security valves operate by venting to the atmosphere, upon power failure of a vacuum pump, the volume above a piston of the security valve whose lower side is at vacuum. The pressure differential across the piston face causes the piston to move, compressing a return spring and closing the passage from the pump to the vacuum chamber. The pump is then vented to the atmosphere through a small orifice in the piston. Upon restoration of power to the vacuum pump, the pump restarts and the venting solenoid closes. The closing of the solenoid causes closing of the vent to the atmosphere, permitting the operating pump to evacuate the volume above the piston through the orifice. When the pressure differential between the volume above the piston and the vacuum chamber becomes sufficiently small, the return spring raises the piston and opens the valve.
In European Pat. No. 148,480 to Paquet et al., a piston is shown having a channel, the channel passing within a stem of the piston. The channel connects the chamber above the piston to the pump when the valve is closed. In this device, the valve is normally closed. No buffer volume is provided between the atmosphere and the piston face having the orifice therein. A diaphragm separates the upper piston volume from a lower piston volume communicating with an orifice. The orifice is in direct communication with the atmosphere. An orifice is disposed above the piston and communicates with the enclosed volume of air above the piston. The valve is controlled by a device which is normally closed.
In U.S. Pat. No. 4,070,001 to Musgrove, a vacuum security valve is shown which includes a bellows which operates to close a port if the pumping system fails, by causing a circular plate to contact the port. A bleedhole extends through the circular plate from the peripheral edge thereof and to the interior surface of the bellows, thereby connecting the interior bellows cavity with the interior valve cavity. Air from the solenoid-controlled vent enters the bellows directly, rather than through an orifice.
A problem with the prior art valves discussed above is that, during closing of the valve upon vacuum pump failure, air flows through the orifice directly into the vacuum chamber. This pressure burst introduces unwanted air into the vacuum chamber. Such unwanted air can potentially damage ongoing processes or experiments within the vacuum chamber, as well as possibly introducing contaminants.
In the prior art devices, it would be desirable to make the single orifice arbitrarily small in order to reduce the gas burst which occurs during closing of the valve. However, the single orifice of the prior art valves cannot be made arbitrarily small to reduce the gas burst because the time required in order to vent the vacuum pump would be unduly increased.